Space Based Augmentation Systems or SBAS are satellite radio navigation systems intended to supplement systems providing a basic satellite navigation system or Global Navigation Satellite Systems GNSS, such as the GPS, GALILEO or GLONASS systems for jointly providing superior performance in terms of location accuracy, availability and continuity of service and integrity of the information provided.
These systems transmit an L-band signal on one or more (typically geostationary) satellites notably carrying a sequence of navigation or Navigation Overlay Frame NOF messages, at the rate of one message per second.
This signal and the transmitted message sequence are defined by an international standards document: RTCA MOPS DO229 currently in edition D “Minimum Operational Performance Standards for Global Positioning System/Wide Area Augmentation equipment”.
The basic architecture of such a space based augmentation system is shown in FIG. 1.
The signal 1 transmitted by satellites 2 of a constellation of a GNSS satellite navigation system is received by a set of ground receiving stations 3, or Ranging and Integrity Monitoring Stations RIMS, dispersed over a wide area (e.g. Europe). These RIMS stations 3 transmit signals 4 corresponding to the signals received via a Wide Area Network or WAN, to a computing centre 5 or Central Processing Facility CPF. This computing centre 5 prepares corrections and integrity data for providing the user with the required performance and at each cycle transmits a part of them in the form of digital navigation or Navigation Overlay Frame NOF messages 6 which are transmitted via the wide area network to a ground transmitting station 7 or NLES, Navigation Land Earth Station. This ground transmitting station 7 transmits signals 8 corresponding to the signals received, to geostationary satellites 9 which rebroadcast them via signals 10 to receivers 11 of users of the service. The users' receivers 11 simultaneously receive these signals 10 and 1 from the generally geostationary SBAS satellites 9 and the signals from the satellites 2 of the GNSS satellite navigation system constellation and each calculate their position with the aid of these two types of signals 1 and 10. It should be noted that the RIMS ground receiving stations 3 also receive these signals 10 and that they transmit the NOF digital navigation messages to the CPF computing centre 5 jointly with the information received from the GNSS constellation satellites 2.
The cycle described above is repeatedly performed by the SBAS space based augmentation system typically every second. Implementation is often carried out as a pipeline, each element during one cycle processing the data to be processed by the next element in the following cycle.
It should be noted that at the level of the user, a NOF navigation message stream sequence of a certain length (typically several minutes) needs to be known for calculating their position. The consistency of the different NOF navigation message streams transmitted successively is therefore a major factor: which is why a NOF navigation message sequence is referred to as being transmitted, and not isolated NOF navigation messages.
Such a basic embodiment does not provide the very short-term availability and continuity expected by users of such a system: a typical availability of the order of 99% and a loss of continuity probability better than 10−5/h are characteristics of the expected performances for currently the most widespread use which is that of civil aviation.
Notably a failure in the main computing centre leads to the transmission of the NOF navigation message stream being interrupted and an immediate loss of continuity with an impact on availability.
SBAS space based augmentation systems with redundancy are known, such as the EGNOS system, as illustrated in FIG. 2.
Several computing centres or CPFs, in this case three CPFs 5a, 5b and 5c respectively receive data signals 4a , 4b and 4c in parallel from RIMS receiving stations and independently prepare their respective navigation message or NOF streams 6a , 6b and 6c which are each transmitted to the ground transmitting station or NLES 7. The ground transmitting station or NLES 7 selects one of these navigation message streams 8 from among message streams 6a , 6b or 6c and transmits it to users' receivers 11, in this case via the intermediary of geostationary satellites. In the event of a failure of one of the computing centres 5a , 5b and 5c , the ground transmitting station 7 or NLES selects the message from one of the other computing centres or CPFs, thus maintaining the continuity of transmission of navigation message or NOF streams to users' receivers. The process implemented by the ground transmitting station 7 or NLES is based on two things:                the choice of computing centre or CPF supplying the navigation message or NOF stream ensuring the best performance for most users. This choice is based on an indicator or information representative of Quality of Service QoS established by each computing centre or CPF and transmitted to the NLES ground transmitting station; and        two hysteresis mechanisms tending to prevent too frequent switching of computing centre from which the navigation message or NOF stream is selected when the information representative of Quality of Service QoS is comparable.        
It should be noted that in this embodiment, the computing centres or CPFs 5a , 5b, and 5c are generally located at different geographical sites and at a distance in order to avoid a complete failure in the event of a major local problem (country-wide network failure, a major industrial accident, natural disaster, etc.).
The EGNOS conventional solution in FIG. 2 presents a major drawback. This is because each computing centre or CPF prepares navigation message or NOF streams, calculation cycle after calculation cycle, according to its own calculations. The message sequence generated by a computing centre or CPF is not the same as that generated by another computing centre.
This is because this message sequence depends on elements which are specific to each computing centre or CPF, even if these computing centres are functionally identical (same hardware and software):                input data: due to the imperfection of wide area networks, different computing centres or CPFs do not receive exactly the same input data. Some messages from receiving stations or RIMS are lost or delivered out of time preventing their inclusion in the current calculation cycle;        their start-up time: the calculations made by a CPF do not depend only on the data of the current cycle but also on intermediate variables, calculated from data received in previous cycles. The calculations of the computing centres or CPFs begin at this start-up time which is determined by operational considerations. These intermediate variables are therefore not calculated from the same history of past data and are therefore not necessarily identical for a given cycle; and        the operational context: for example, for determining the SBAS time (virtual time acting as a common reference for all calculations in a CPF (ENT—EGNOS Network Time) in the case of EGNOS), the computing centre or CPF considers the clocks of some receiving stations or RIMS, at its start-up. If the network of receiving stations or RIMS available at the start-up of a computing centre is different from that corresponding to another computing centre, these two computing centres are synchronized on a virtual time based on a set of different clocks.        
Because of the difference between these NOF navigation message sequences, when switching between the message sequence generated by a computing centre and that generated by the computing centre chosen by the NLES ground transmitting station for taking over, the problem arises of maintaining consistency between these message sequences.
In a system such as EGNOS, this has involved the need to introduce margins in some key parameters for preventing essential performance degradation (such as integrity) due to this switching between two sequences of independently generated messages. These margins reduce the normal performance of the system outside periods of switching between computing centres.
Another drawback is associated with current systems seeking to minimize this very unfavourable effect. This is possible by means of:                the use of a high quality wide area network, itself implementing redundancies in data transmission, to the detriment of the cost of the system. Typically the use of a private network may be necessary for obtaining high quality transmission, difficult to achieve on commercial networks shared with other users;        the use of a long period of convergence of calculations of a computing centre after its initialization before using the outputs of this computing centre in the operational system, enabling it to regain a state closer to that of the computing centres started up before it. The computing centre is then declared operational and is selectable by an NLES ground transmitting station. This approach leads to operational difficulties, restarting a computing centre taking a significant time detrimental to the operation and efficient maintenance of the system. Typically, the design of the EGNOS system provides for a convergence time of 72 hours.        